A HVAC system typically includes a compressor, a condenser, an expansion device and an evaporator, forming a refrigeration circuit. In a cooling cycle, refrigerant vapor is generally compressed by the compressor, and then condensed to liquid refrigerant in the condenser. The liquid refrigerant can then be directed through the expansion device to reduce a temperature and become a liquid/vapor refrigerant mixture (two-phase refrigerant mixture). The two-phase refrigerant mixture can be directed into the evaporator to exchange heat with, for example, air moving across the evaporator. The two-phase refrigerant mixture can be vaporized to refrigerant vapor in the evaporator.
Some HVAC systems may be able to operate in a heating cycle. These HVAC systems are typically called heat pumps. During a heating cycle, the process is generally reversed from the process in the cooling cycle. In the heating cycle, the evaporator in the cooling cycle functions as a condenser, and the condenser functions as an evaporator. After being compressed by the compressor, the compressed refrigerant vapor is typically directed to the evaporator first so as to release heat to, for example, the indoor air, which also condenses the refrigerant vapor to liquid refrigerant. The liquid refrigerant is then typically directed to the condenser through an expansion device to become a two-phase refrigerant mixture.
The evaporator and the condenser are heat exchangers. Heat exchangers are typically configured to help establish a heat exchange relationship between a first fluid (such as refrigerant and a process fluid) and a second fluid (such as air). Various types of heat exchangers have been developed to work as a condenser and/or an evaporator. One type of heat exchanger is a micro-channel heat exchanger (MCHEX). A typical MCHEX may include micro-channel tubes running in parallel between two headers. The adjacent tubes generally have fan-fold fins brazed therein between. The micro-channel tubes form fluid communication with the headers. Refrigerant can be distributed into the micro-channel tubes from the headers, and/or collected in the headers when the refrigerant flows out of the micro-channel tubes. Outer surfaces of the micro-channel tubes and the fins may help heat exchange between the first fluid (such as refrigerant) in the micro-channel tubes and a second fluid (such as air) flowing across the outer surfaces of the micro-channel tubes.